I think this is a long shot for several reasons:
1. Cruise ships have a short life span. Most ships are sold after about 20 years to a lower quality cruise line. This is because the older the ship gets, the more costly it is to maintain. HAL, Celebrity, and Princess all do this.
Disney would never keep a ship on line that was not up to par.
2. The Maintenance cost invloved in keeping a ship in good repair is very high. Cruise lines cover these costs thru regular full price bookings. Maintenence fees would not be high enough to cover this cost, or would be so high as to make it cost prohibitive.
3. A large staff would have to be paid to run the ship. Again this would cost a fortune.
4. The main appeal of cruise ships is that the meals are all included. Would your
DVC membership include meals for the next 20 or so years?
I just don't see how this would fit into a DVC model. If they charged you enough it might work, but I bet it would just be cheaper to book a regular cruise than pay the inflated cost of a "Floating Timeshare" I think the cost would be so high that there would not be a market for it.